Maternal smoking and fetal carboxyhaemoglobin and blood gas levels

Introduction Maternal smoking is associated with about a 5 YO reduction in fetal growth which represents 171 g at term1,'. Although this may seem only a modest decrease, it is of great importance to public health because perinatal morbidity and mortality (especially related to fetal hypoxia) are increased as the birthweight distribution is shifted downwards3. The association between smoking and reduced birthweight is thought to be causal, because it is found in a wide variety of study designs, exhibits a dose relationship and has biological plausibility4v5. Indeed, smokers who give up at any time before 30 weeks gestation have heavier infants than those who persist beyond6. As well as reducing fetal growth, maternal cigarette smoking has serious adverse effects on health, both before and after birth, and implications for the subsequent development of the infant and child. There are established associations between smoking in pregnancy and spontaneous abortion', placental abruption', premature delivery' and postnatal deficits in growth and intellectlo-". Further, there is increasing evidence that growth restriction in utero has implications for disease in later life12. Despite the above epidemiological evidence, little is understood about the mechanisms by which smoking influences fetal growth. A better understanding of this is not just of academic interest. When informed many pregnant women are not impressed that a reduction in their babies' birthweight of 171 g is of any great importance and health education programmes have been less successful than hoped. If we could explain why smoking damages fetuses, the medical profession might be more successful in conveying this message. Reducing smoking during pregnancy would decrease one of the most im-